The present invention relates to a reflective liquid crystal display device with a reflection electrode.
Liquid crystal display devices (LCD) are thin and light thus have been used for a wide range of purposes such as a display for a mobile information terminal. LCDs do not emit light themselves. They are a light-receiving-type device which displays an image by changing the level of transmittance of light. LCDs can be driven by effective voltage of several volts. Therefore, if LCD is used as a reflective-type by disposing a reflecting plate under the LCD to utilize reflection of external light, no electricity is required for a back light, thus the display device becomes extremely efficient in terms of power consumption.
A conventional reflective color LCD comprises a liquid crystal cell including color filters, and a pair of polarizing films which sandwich the liquid crystal cell. The color filters are disposed on one of substrates of the liquid crystal cell. On top of the color filters disposed on the substrate is a transparent electrode. By applying voltage on the liquid crystal cell, orientation of the liquid crystal molecules is changed, thereby controlling the transmittance of light of each color filter to display a color image.
The transmittance of a polarizing film is only about 45% at the maximum. The transmittance of the polarized light parallel to the absorption axis of the polarizing film is approximately 0% while that of the polarized light perpendicular to the absorption axis of the polarizing film is 90%. In the case of the reflective LCD using two polarizing films, light goes through polarizing films four times before emitted. Therefore, the maximum reflectance can be defined as:
xe2x80x83(0.9)4xc3x9750%=32.8%
when absorption by other materials such as a color filter is not considered.
Thus, even the reflectance of a black and white display panel which does not use color filters is only about 33%. If color filters are applied to such a device to display a color image, the reflectance will be reduced to about one third of the original reflectance, inhibiting the achievement of a reflectance high enough for a sufficient luminance.
To brighten the display, several constructions have been proposed, for example, by the Japanese Patent Application Unexamined Publications No. H07-146469 and No. H07-84252, in which only one polarizing film is used on top of the liquid crystal cell, which is sandwiched between the polarizing film and a reflecting plate. In this case, light goes through the polarizing film only twice, thus the maximum reflectance can be defined as:
(0.9)2xc3x9750%=40.5%
when absorption by other materials such as the color filters is not considered.
Therefore, in this case, a maximum increase of 23.5% in reflectance from the construction using two polarizing films can be expected.
However, with this reflective LCD with one polarizing film, when displaying a color image by using color filters while trying to obtain higher luminance by increasing the reflectance, color drift often occurs, obstructing a clear achromatic display. In particular, an achromatic black has been difficult to display.
The Japanese Patent Application Unexamined Publication No. H06-308481 has disclosed a reflective color LCD which displays a colored image without color filters by utilizing birefringence of the twisted nematic liquid crystal layer and polarizing films. The Japanese Patent Application Unexamined Publications No. H06-175125 and No. H06-301006 have disclosed a color LCD which utilizes birefringence of a liquid crystal layer and phase retardation films. Since these LCD do not use color filters, a reflectance high enough to achieve practical luminance is ensured even when two polarizing films are used. However, in the case of the foregoing devices, since a colored image is displayed based on coloring by birefringence, multi-gradation, multi-color displays such as 16 gradation, 4096 color display and 64 gradation full-color display have principally been difficult. Moreover, the color purity and the color reproduction range have been limited.
Considering the aforementioned issues, the present invention aims at providing a reflective LCD that achieves a bright white display and a high contrast, and is capable of displaying achromatic black and white as well as multi-gradation and multi-color displays.
To achieve the foregoing purpose, the present invention has the following construction.
The reflective liquid crystal display device of a first construction of the present invention comprises the following elements;
a) a liquid crystal cell formed of a nematic liquid crystal injected between a first and a second substrates;
b) a polarizing film disposed on the first substrate of the liquid crystal cell;
c) two retardation films disposed in between the polarizing film and the liquid crystal cell; and
d) a light reflecting means disposed on the second substrate. The twist angle of the nematic liquid crystal xcex94LC is 45xc2x0xcx9c90xc2x0. The product of the birefringence of the nematic liquid crystal xcex94nLC and the thickness of the liquid crystal layer dLC, namely xcex94nLCxc2x7dLC is 0.20 xcexcmxcx9c0.3 xcexcm. The retardation value of the retardation film on the polarizing film side RF1 is 0.23 xcexcmxcx9c0.28 xcexcm. The retardation value of the retardation film on the liquid crystal cell side RF2 is 0.13 xcexcmxcx9c0.18 xcexcm. The Z coefficient QZ of each retardation film is 0.3xcx9c1.0. When bisector of the larger of the angle formed by the orientation direction of the liquid crystal molecules closest to one of the substrates and the orientation direction of the liquid crystal molecules closest to the other substrate, is set as a reference line in the substrate, and when the direction in which the nematic liquid crystal is twisted from the first substrate to the second substrate, viewed from the side of the first substrate, is defined positive, the present invention is featured by
ØP is 75xc2x0xcx9c195xc2x0, ØPxe2x88x92ØF1, 105xc2x0xcx9c115xc2x0, and ØPxe2x88x92ØF2, 165xc2x0xcx9c175xc2x0,
where an angle formed by the reference line and the direction of the absorption axis of the polarizing film is ØP, an angle formed by the reference line and the direction of the slow axis of the retardation film on the polarizing film side is ØF1, and an angle formed by the reference line and the direction of the slow axis of the retardation film on the liquid crystal cell side is ØF2.
The aforementioned QZ is a coefficient defined as:
QZ=(nxxe2x88x92nz)/(nxxe2x88x92ny)
where nx, ny and nz are refractive indices of each retardation film in the directions of each axis of spatial coordinates (x, y, z) in which the z axis is a direction normal to the retardation film, the x and y axes are parallel to respectively the slow axis and the fast axis of each retardation film. nx is a refractive index in the direction of the slow axis and ny the fast axis of each retardation film.
According to the first construction, a bright normally-white reflective liquid crystal display device displaying achromatic black and white can be achieved.
In the case of the first construction, the angle ØP formed by the reference line and the absorption axis of the polarizing film is preferably 90xc2x0xcx9c120xc2x0 or 150xc2x0xcx9c180xc2x0. This preferable construction achieves even higher contrast and better display properties.
A reflective liquid crystal display device of a second construction of the present invention is based on the first construction and is featured by
ØP isxe2x88x9215xc2x0xcx9c105xc2x0, ØPxe2x88x92ØF1, xe2x88x92105xc2x0xcx9cxe2x88x92115xc2x0, and ØPxe2x88x92ØF2, xe2x88x92165xc2x0xcx9cxe2x88x92175xc2x0.
According to the second construction, a bright normally-white reflective liquid crystal display device, also displaying achromatic black and white can be achieved.
In the case of the second construction the angle ØP formed by the reference line and the absorption axis of the polarizing film is preferably 0xc2x0xcx9c30xc2x0 or 60xc2x0xcx9c90xc2x0. This preferable construction achieves even higher contrast and better display properties.